Bruce, I believe that your suspicion is correct. The sticking point in the design of a high performance up-conversion receiver at this time is the first LO, whose phase noise must be suitably "low" and whose cost is acceptable. There can be problems with VHF "Roofing" filters, but there are ways to overcome these.
Regarding the use of very narrow roofing filters (crystal) in a down-conversion receiver, there is an underlying filter generated IMD problem which might or might not affect the IMD performance of the overall receiver - depending upon the "IMD performance" of those parts of the receiver ahead of and behind the filter. The problem is that for any given quality of quartz used in the crystals, the IMD performance of a crystal filter can be shown to worsen as the filter's bandwidth narrows. I suspect that the 6 kHz roofing filter used in Kenwood's TS-590S ahead of the IF crystal filters is there to give some protection to the narrower IF filters. 73, Geoff GM4ESD Bruce McLaughlin wrote on Wednesday, November 17, 2010, at 11:02 PM: >I suspect it is going to become the standard for many if not most of the >new > radios. As soon as it became apparent that effective roofing filters can > really improve the close in IMD performance, it seems as if a low first IF > frequency has become almost mandatory. I note that the Orion and the new > Yaesu FT 5000 both use down conversion for the first IF and, therefore, > have > effective narrow roofing filters. I wonder when, or if, ICOM and the > others > will follow suit. <snip> ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
